SCCM 2012 / 2007 Active Directory (AD) Domain Functional Level for SCCM / Configuration Manager?

I had a question what should be the Active Directory Domain Functional Level Configuration Manager?

It actually required AD schema extended for below benefits;
Network Access Protection (NAP): - SCCM publishes health state references to Active Directory Domain Services so that the System Health Validator point (SHV) for validate the client’s statement of health.

For AD Domain Functional Level there is no minimum requirements however, as part of AD System Discovery if you wanted to get the last system logon information you required at least windows 2003 server.

See below ms documentation for more information https://technet.microsoft.com/en-us/library/hh368980.aspx